Cannon, 33, offered testimony Tuesday at a public hearing at the State House in support of a sweeping bill to toughen the state’s gun laws. The measure, unveiled last week by House Speaker Robert A. DeLeo, contains more than 40 provisions that include expanding police discretion of a person’s “suitability” to own a gun and stepping up scrutiny of private sales.

A large contingent of gun-rights activists pushed back against the bill at the packed hearing before the Joint Committee on Public Safety and Homeland Security, saying that the bill infringes upon the rights of lawful gun owners and that it will not reduce crime. Many of the few hundred in the audience wore stickers or carried signs in support of advocacy groups.

Representative George Peterson Jr., Republican of Grafton and the assistant House minority leader, said the bill gives an impression that lawful gun owners are the problem, rather than aiming at street crime. In particular, he criticized the clause that would give police discretion to deny a permit for a rifle or a shotgun if an applicant is deemed unsuitable.

“I have never committed a crime. I have never been treated for a mental illness. I have never been treated for alcoholism,” Peterson said. “But I might, under the way this bill is proposed, be ineligible because I don’t know what the suitability standards will be.”

He asked for a standard defined in law, a requirement that police state the reason for license denial in writing, and an appeals process.

“A lot of chiefs just refuse to issue licenses because they don’t want their constituents to have guns,” he said.

Peterson received raucous applause from gun-rights supporters, as did Jim Wallace, head of the Gun Owners Action League, who also criticized the suitability clause.

“To expand something that has been so widely abused is simply unconscionable,” he said. “The burden must be on a public official who wants to remove my civil rights.”

Wallace urged the Legislature to revisit the bill, which he said was not nearly hard enough on street crime. He supported parts of it, such as having Massachusetts join a national database for criminal and mental health background checks.

Attorney General Martha Coakley told the panel that the bill addresses the critical intersection of many issues: gun violence, suicide prevention, mental health care, domestic violence, and school safety.

“The fact that we’ve seen tragedy after tragedy and yet the federal government has still failed to pass a federal assault weapons ban or any firearms legislation on the national level is unfathomable,’’ said Coakley, who is seeking the Democratic nomination for governor.

State Treasurer Steven Grossman, a Democrat who is also running for governor, also spoke in favor of the bill. “Gun laws save lives,” he said.

The bill could be the most comprehensive update to state gun laws in 16 years. DeLeo has said gun-violence prevention cannot be put on hold while awaiting federal action, citing the 2012 Sandy Hook Elementary School shooting as a catalyst for the legislation. DeLeo has said he hopes for final passage of the bill by the end of the legislative session July 31.

Representative Jeffrey Sánchez, a Jamaica Plain Democrat, told the story of a childhood friend who was fatally shot near the Longwood medical area just five weeks ago.

“People are dying in streets all over this Commonwealth relative to gun violence,” Sánchez said.

Massachusetts has the nation’s second-lowest rate of gun-related deaths, but homicides committed with firearms have been on the rise since passage of the state’s last comprehensive gun-control package in 1998. In 2011, there were 122 such homicides, an increase from 65 in 1998.

Cannon, the Roxbury mother, said she knew five young people, including the son of a friend, who have been killed in the neighborhood in the past five years. “You get to see firsthand what a mother goes through when her child is murdered,” she said in an interview.

Earlier, she asked the audience: “How long do I have before my son is tired of running?”